N-Heterocyclic carbenes (NHC) and parent cations (NHCH+): role of the scaffold on the acidity, nucleophilicity, stability and hydrogen bond. An electrochemical evaluation and ab initio calculations.

N-Heterocyclic carbenes (NHCs) are widely used as organocatalysts. Their reactivity (and instability) is related to their basicity and nucleophilicity, which, in turn, are linked to their scaffold. NHCs can be generated by chemical deprotonation or electrochemical reduction of the parent azolium cations, NHCH+s. Cyclic voltammetry enabled the reduction potential of the NHCH+s to be determined; the reduction potential is related to the acidity of the NHCH+s and the oxidation potential of the NHCs, which is related to the nucleophilicity of the NHCs. It was thus possible to order different NHCH+s and NHCs by their acidity and nucleophilicity, respectively. A study on the stability of NHCs was also performed in the absence and in the presence of acetic acid to assess the possibility of the coexistence of NHC and an acid in the same solution, opening the possibility of co-catalysis. Finally, ab initio calculations confirmed the presence, in DMF, of hydrogen-bonded NHCH+ –NHC adducts, which could influence catalyst activity.